* Represents a function that accepts one argument and produces a boolean result.
 * @param <T> the type of the input to the function
 */
@FunctionalInterface
public interface BooleanFunction<T> {
    /**
     * Applies this function to the given argument.
     * @param t the function argument
     * @return the function result
     */
    boolean apply(T t);

    @Test
    public void test_apply_string() {
        BooleanFunction<String> isNotEmpty = s -> s != null && !s.isEmpty();

        assertTrue(isNotEmpty.apply("test"));
        assertTrue(isNotEmpty.apply("a"));
        assertFalse(isNotEmpty.apply(""));
        assertFalse(isNotEmpty.apply(null));
    }

    @Test
    public void test_apply_integer() {
        BooleanFunction<Integer> isPositive = n -> n != null && n > 0;